1. Field of the Invention
Water-dilutable two-component epoxy resin systems which cure at room temperature have reached a technological level which comes close to conventional epoxy resin systems with respect to the desired properties such as adhesion to substrate, surface quality, and mechanical and chemical resistance.
Attempts were made, for convenience and processing safety, to develop monocomponent (1C) epoxy resin systems with latent catalysis, consisting of a combination of ketimines and epoxy resins. The advantages of these 1C systems are obvious: the hardener and epoxy resin are already present in the proper proportions, so that there is no danger of using an incorrect proportion. In conventional systems, curing occurs when the ketimines are hydrolyzed by moisture from the air. With water-dilutable systems, it is preferable to add a definite quantity of water, so that the curing time no longer depends on the humidity. Moreover, this leads to homogeneous curing through the entire film thickness, thereby improving the properties of the cured resin.
2. Description of the Related Art
Conventional 1C systems have been known for a long time, and are described in the following patents, for example: U.S. Pat. Nos. 3,442,856, 3,919,317, 4,148,950, 4,391,958, 4,748,083 and WO Application 92/18575.
Water-dilutable 1C systems with latent catalysis are described in WO-A 98/02478. A water-dilutable liquid epoxy resin is combined with ketone-blocked amines, known as ketimines. When water is added, the ketimine decomposes and the amine hardener is regenerated and is available for curing. As the system can be emulsified in water, this is an aqueous binder, the properties and processing of which are known to users of the two-component aqueous systems. This binder assures easier processing for the user, as the binder is already present at the correct ratio of hardener to epoxy resin. The curing reaction is started by addition of water, so that only a minimum quantity of water need be added to attain optimum properties for the coating.
Suitable amines which react with ketones to give ketimines include aliphatic linear, branched or cyclic amines with at least two primary amine groups, primary polyoxyalkylene diamines, partially or completely hydrogenated condensation products of aniline or its derivatives, as well as polyamidoamines with at least two terminal primary amino groups from aliphatic linear, branched or cyclic amines with at least two primary amino groups and dicarboxylic acids.
These amines have disadvantages, though. For instance, low molar mass diprimary amines combined with water-dilutable epoxy resins hardly give the desired properties such as good surface quality and a balanced ratio of film hardness to film flexibility after curing. Although polyoxyalkylene diamines are of high molar mass, their aliphatic and polar nature produces insufficiently water-resistant films. Condensation products of aniline derivatives, even if completely hydrogenated, are not very reactive at ambient temperature, which means that the curing time is long.
Non-hydrogenated, or only partially hydrogenated, products are also toxicologically objectionable. Amine hardeners for ketimine capping should contain solely primary amino groups, because only those can react easily with ketones, with elimination of water. Reaction of secondary amines with ketones to give enamines proceeds less smoothly. A residue of unblocked amines remains, seriously degrading the storage stability of the 1C systems. For that reason, epoxide-amine adducts and Mannich bases are not very suitable for ketimine capping.
Polyamidoamines meet the requirement that they contain only primary amino groups, and they also have the desired polymeric structure. One disadvantage of the less polar polyamidoamines is their poor compatibility with the water-dilutable epoxy resins. That is particularly the case if the dicarboxylic acids used have long chains (10 or more carbon atoms in the chain). Polyamidoamines based on short-chain dicarboxylic acids, on the other hand, are often crystalline and high-melting, and thus difficult to work with.
Poor compatibility of ketimines based on polyamidoamines with water-dilutable systems becomes apparent to the user primarily through surface problems and cloudiness in unpigmented paint films in the cured systems. The problem can, indeed, be reduced by adding suitable solvents; but that impairs the solvent balance of the system. Also, the advantages of an aqueous 1C system are partially lost.